Telecommunications customers with a high volume of communications between specific sites have long used dedicated leased lines between those sites. Tariffs for those leased lines are based on the line' length and their maximum bit rate. The advent of optical telecommunications, based on laser sources and optical fiber cable, has been accompanied by similar leased-line service offerings. One popular option is called "dark fiber". In this service the communications company provides an optical fiber link, but none of the equipment at either end. The customer leasing the optical fiber link decides the nature of that equipment. Thus, the communications company supplying the optical fiber link knows the distance covered by the link. However, the communications company does not know the maximum bit rate being utilized by the customer.
In order to establish a maximum usable bit rate, and thus a rational basis for tariffing such optical fiber links, some device for limiting the maximum bit rate of such links would be useful. One such device is known as a bit-rate limiter (BRL). A BRL is composed of a four-port (2.times.2) directional coupler which is inserted into the optical fiber link. Illustratively, to insert the bit rate limiter, the fiber link is cut to form first and second cut ends. One cut end of the link is connected to the first input port of the coupler and the second cut end is connected to the first output port of the coupler. To provide a recirculation path, an optical fiber loop is connected between the second output port and the second input port of the coupler. This optical fiber loop functions as a delay line.
The fraction of the power which is coupled from the first input port of the coupler to the second output port (or from the second input port to the first output port) is referred to as the cross-coupling ratio k. The fraction of the power that passes straight through from the first input port to the first output port (or from the second input port to the second output port) is (l-k). Thus, the cross-coupling ratio k determines how optical power arriving via one of the input ports is distributed to the two output ports.
Consider an input pulse arriving at the coupler via the optical fiber link and the first input port. A fraction (l-k) of the pulse power passes straight through to the first output port and a fraction k of the pulse power enters the recirculation path via the second output port. This latter fraction recirculates via the fiber loop delay line and reenters the coupler via the second input port wherein a fraction is returned to the main fiber link via the first output port and a fraction is recirculated again. This process repeats so that the net effect is that a pulse propagating on the optical fiber link and entering the coupler via the first input port is broadened when it leaves the coupler at the first output port to resume propagating via the optical fiber link. The extent of the pulse broadening sets an upper limit on the bit rate that can be transmitted by the fiber link. In particular, the longer the fiber delay line, the greater the pulse broadening and the lower the limit on the maximum bit rate.
More analytically, the operation of the BRL may be understood by considering its transfer function. In particular, the transfer function has a zero at f.sub.o =1/2.tau. where .tau. is the delay of the optical fiber loop. This means that only signals having a bit rate below f.sub.o pass undistorted through the BRL, thereby establishing a bit rate limit for the optical fiber link.
The above-described BRL device provides a simple and effective method of bit rate limitation, but it imposes the identical bit rate limitation on all the signals passing through the optical fiber link. Optical fiber links have a very high inherent bandwidth capacity so that they are frequently used to simultaneously carry several wavelength channels. Typically, some of these channels may be used for low bit-rate applications, and others for high bit-rate applications, such as high-definition digital video.
It would be useful to have a device for achieving an individual bandwidth limitation for each individual channel of a multi-channel optical fiber link. Accordingly, it is an object of the present invention to provide a device for affecting a desired bit rate limitation for each of the channels in a multi-channel optical fiber communications link.